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>43.4. Visibility of Data Changes</A
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><P
>   The following rules govern the visibility of data changes in
   functions that use SPI (or any other C function):

   <P
></P
></P><UL
><LI
><P
>      During the execution of an SQL command, any data changes made by
      the command are invisible to the command itself.  For
      example, in:
</P><PRE
CLASS="PROGRAMLISTING"
>INSERT INTO a SELECT * FROM a;</PRE
><P>
      the inserted rows are invisible to the <TT
CLASS="COMMAND"
>SELECT</TT
>
      part.
     </P
></LI
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>      Changes made by a command C are visible to all commands that are
      started after C, no matter whether they are started inside C
      (during the execution of C) or after C is done.
     </P
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>      Commands executed via SPI inside a function called by an SQL command
      (either an ordinary function or a trigger) follow one or the
      other of the above rules depending on the read/write flag passed
      to SPI.  Commands executed in read-only mode follow the first
      rule: they cannot see changes of the calling command.  Commands executed
      in read-write mode follow the second rule: they can see all changes made
      so far.
     </P
></LI
><LI
><P
>      All standard procedural languages set the SPI read-write mode
      depending on the volatility attribute of the function.  Commands of
      <TT
CLASS="LITERAL"
>STABLE</TT
> and <TT
CLASS="LITERAL"
>IMMUTABLE</TT
> functions are done in
      read-only mode, while commands of <TT
CLASS="LITERAL"
>VOLATILE</TT
> functions are
      done in read-write mode.  While authors of C functions are able to
      violate this convention, it's unlikely to be a good idea to do so.
     </P
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  </P
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>   The next section contains an example that illustrates the
   application of these rules.
  </P
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